Automatic control of electric arc furnaces



Aug. 30, 1949. L. GRANGETTE 2,480,453

AUTOMATIC CONTROL OF ELECTRIC ARC FURNACES Filed Feb. 28, 1946 3 She ets-Sheet l Aug. 30, 1949.

L. GRANGETTE AUTOMATIC CONTROL OF ELECTRIC ARC FURNACES Filed Feb. 28, 1946 3 Sheets-Sheet 2 Amp.

0, 1949. L. GRANGETTE 2,480,463

AUTOMATIC CONTROL OF ELECTRIC ARC FURNACES Filed Feb. 28, 1946 3 Sheets-Sheet 3 9 NM mm m SP1 I N VEN TOR. langel'h A W J W W T a w 3 I \w T Q WEN a /r \m 9 Nb mm ffi\ Mg Patented Aug. 30, 1949 AUTOMATIC CONTROL OF ELECTRIC ARC FURNACES Louis Grangette, Casablanca, Morocco Application February 28, 194.6, Serial No. 650,963 In France July 18, 1939 Section 1, Public Law 690, August 8, 1946 Patent expires July 18, 1959 2 Claims. 1

During the running of an electric arc furnace and in particular of a furnace for steel, the intensity of the arc current is subjected to sudden and sometimes considerable variations, due to the wear of the electrodes, variations in the level of the bath and of the charge, and also to the agitation which is produced in the liquid bath.

These variations obviously react on the potential at the terminals of the furnace and cause disturbances in the mains which are always very troublesome and may be dangerous either for the transformers or for the alternators which produce the current. In any case, the power absorbed is modified and the specific consumption of current per ton produced is increased.

Now, it is important industrially, to avoid these disturbances and to reduce to a minimum the specific consumption. The first conditon requires that the intensity and the potential should be kept as constant as possible and the second, that the power factor or cos be as near unity as possible, whilst giving to the power absorbed a value sufficient for the operation to be rapid and the external losses be reduced to a minimum.

It is possible by known devicesinsulated and closed iurnaces-to considerably reduce the external losses in such manner that, in the choice of the running energy, one can be guided above all by the necessity of making cos it close to unity, whilst ensuring a sufficiently rapid production.

It therefore appears that the best method of regulation would consist, presuming that the absorbed power best adapted to the furnace for obtaining a low specific consumption has been determined, in keeping this power absorbed constant.

Unfortunately, the regulation to constant power absorbed is not at present always possible, principally in a case of furnaces with great inductance, such as steel furnaces, at least if it is desired to regulate it simply by displacement of the electrode. This phenomenon is well known and it has been noticed that, in certain cases, when the furnace runs with the full charge, if the potential at the terminals of the mains falls below a certain value, the production of the furnace drops, in spite of any increase of the intensity allowed in the are by the displacement of the electrode. The more amperes the furnace absorbs, the more the specific consumption of current increases, whilst at the same time the hourly production of the furnace diminishes; this is due to an excessive reduction of the cos Regulating apparatus have been used acting on the electrodes and having for their purpose either to keep the potential constant or to cause the furnace to operate with a constant length of arc, or again at constant impedance. Whilst all these apparatus ensure the desired regularity of 2 running, they have the great drawback of sacrificing to this the specific consumption.

The device according to the present invention presents a considerable improvement and effects a very important economy because it ensures the regulation of the running of the furnace with constant power absorbed in every case.

There is thus obtained perfect regularity of running at the same time that the specific consumption is reduced to a. minimum, since the device allows of fixin the value of the power absorbed as desired, that is to say obviously at the figure which will give the minimum of losses.

Referring to the drawing:

Figures 1-3 are graphs referred to in the mathematical discussion.

Figure 4 is a schematic representation of one form of the invention applied to a single phase furnace.

Figure 5 is a modification of the single phase furnace of Figure 4.

This device comprises, on the one hand, a minimum potential regulator constituted by a voltmetric relay acting on the supply voltage of the arc and, on the other hand, a regulator of power absorbed constituted by a wattmetric relay acting on the distance apart of the electrodes.

The voltage regulator and the power regulator may be connected in such manner that they cannot function simultaneously. Preferably, in this connection, the voltage regulator controls in a preponderant manner the circuit placing the power regulator in operation, so that the power regulation is only possible if the voltage is above a limiting value.

According to a modification of the device, the voltage regulator is both minimum and maximum and the power regulator then comprises a simple ammeter relay.

The justification for the efiiciency of the device according to the invention is found in the mathematical reasoning shown hereinafter with reference to Figs. 1, 2 and 3 of the accompanying drawings.

The known formula which connects the voltage E at the terminals of the arc, the current intensity I traversing the latter, and the distance 2 of the electrodes from each other is as follows:

a, b, c, It being constants This can be represented graphically (Fig. 1) by a series of hyperbolas, all located above the straight line A, the equation of which is:

These hyperbolas all admit for a common asymptote the axis OE of the ordinates and as 3 d-asymp e s n e teee s l e eequation?" W (2) E=a+bzI passing through the point'iyhere the straight line A intersects the axis OE of the ordinates.

On the other hand, the power factor cos o, is 7 given by the formula:

(3) cos q5=' /1--L w in which L represents the inductance ofthe, furnace circuit and w the frequency of the current. For a value E of the voltage at the terminals of the furnace, the useful power P=EI cos could ,be'kepticonstant iby shiftingzthe electrodes if the quantity.l cos walwaysivaries in the. same'sense as the; current sizitensitysswhenz the ;.electrode is V V. y .r,

,Thequantityl cos .may bewritten as But I is relatedto 2 by the Equation i which m be 3341 31 3 3 re sbr ree iet ee et is the same Thesecondrfactoris positive. v

eat. The dether beq ewn o the ra h-re resent ve this straight line passes through the origin of the co-ordinates aEiid-dividesthe plane into two re gions.

When the point representative, of the state of the furnace is'in' the 'regionlocated below the straight line OBXregion etchedon" Figure 2) the derivativei.(4)eisip6sitive. rrtnen the separation z of the. electrodes-is diminished (whichhas'for effect to increase I) the useful power decreases. Now, the representativepoint has all the greater I pr ab l trof eq pes nte he.sa s1; tc ed region,

th ets teeslerse es. e m n lw i I ,straightsline OB hasithat is topay, the greater the inductance 'of the furnace.

It is, of course, found in practice that the greater the inductance of the circuit of a furnace i is,-the-easier there is produced the phenomena "termed sluggishness of the furnace.

si sts in a slowing down of the production and an This conincrease in the specific consumption, when it is soughtto compensate for a lowering of the voltage *a'tthe'terminals of the furnace by increasing the current intensity admitted in the are by ap- {proaching the electrodes.

If, on the contrary, the point representative of the-state of the furnace is in the zone located above the straight line OB represented by the Equation 5,1 thelpowertincreases when the elec- "'trodes -.are1 approached; that late-say; when: the

, .curr'ent intensity isincreased I If the point-representativeof the stateiotthefurnacecan'be kept in this zone, it is then always-possible to make the regulation by constantspower -absorbed, -acondition ,for the best output.

Examination .of the-graphpermits of perceiving by what process it is possibleto keep the point representative ohthe state of' the furnace in the most advantageous-zone, and of -efiecting at the same time regulation byconstant power absorbed. Let Z,be the maximumliftofthe furnace elec- -trode. Let us consider the corresponding hyperbola (Figure3) representing the thecurrent passing .in:the are and=trace-the straight line fiWQW If this straight-line does not intersect-the hyper- .bola (2) it is evidentthat for every value ofthe ivoltage. permitting the passage of the current in the are for this lift Z of:the'electrode-the con- 7 stantspower regulation is-possible,=-since all the points of. the hyperbolaare above'the straight line. For it tobe so, it is sumcient that the angular coefiicient'bZpfi the oblique asymptote of-the whyperbolalZ) be greater-than the angular coefficient iiLw ofthe straight line" QB, "that is to ,say' that:

7 We then-trace throughlAathe parallel AD to -the. straight line: e=I\/ 2 Lw. This --paralle1 is asymptote to a hyperbola zoom-responding to a lift 20 less than the maximum lift of the electrode.

For, any lift less thanszo,"thereexists a-value of z the yoltagessuchthat'if thesvoltage at the teraminals of theifurnace is less than this it lsno long r nofisibletorkeen the useful'spower constant yeim lylman pu atinethe electrode; This value of the voltage ;i hat: which corresponds to.:.the me ing oi t;QoL-thestraight. linevOB.

, eiecosr (pi-B.

the constant valueP being the powerv which has been chosen-'a'sregulating power giving the lowest specific-consumption bf burren t.

Th'eh leti be a'nyposition Whatever of. the e1ec- V trode comprised b'etweenthe lift 20 and the contact with the base-and 1et U-be -the;vo1tage corresponding thereto at the terminals of the furnace,; for. this-Lposi'tion ofsthe electrode, at-the optimum portenBchosenxnL'I-h state of running QfsQ Q furnace, r l hcn; representeds vbyJthe point n r eqtiqncommonatethe straightline S to the hyperbole,-:representatlveniithe:aro'fonthe lift'ziofr the electrodeend to the-:cur'vei l se e Let us supposezsthatrlintconsequence prover: chargeofthe mains-1011101 any othemreasonrthe. voltagelof .theiurnaces. diminishes-and a sum s the .lvaluezUi. Thepoint representatives oLthe;

stateaof therifurnace comeato Milithelnoin -ofinr;

tersection-n or. the, hyperbole ,(Zl)-.. ,andazotz the straight line..e,-.-=U-1.

If' it is desired,tcmamtainathetu-s imanower at! .its:value.z P; it. is.- nece.ssarystolcwer therelecr odeito a:.-po.sitionvsuchztba s he neint -re r -i sentative of the state of the furnace omes to on t e curveJ'e .Butthesrepre entat point-ca o y ometo Mal fl sipoint -Mzzh inters ct n i 'the strai l ne. re- 1;en eofthechvne bolic; ve F is a QVethest a. ht lineDB; tisgnec: s y o histh t if G. i t noin coflmters eonlointh s a ht. ne 0. an the-h p rb l I. .lGz othe S ra ght 1 on i-thetvo t ee i. t t rmina he nn te- 'That i to a t t. e; el eseat thel e nmals o the furnace must not drop below the value V corre pe i to. he-o dinate,-r it eli ointfi a o fin d s the ritieahvaue-p ithelv tae or s ond net e.z ee e ienr ewenf hosem. he d ees ttheiurnee c rc itend i th syolta e r a h i dt lr ui ei xeo stee owe i is then; indispensable, 11 t-of a1 -l to brlng the;

voltage at the terminal ofi th furn ace,,-above e c tica e esc te. iQ Q=:1 ;lS:PQ .t a Shifting. t e eetro ..-.t ke ni hemwen e; sorbed constant.

It i t sh w w e he-abat .eemen tratio that. e u eishees il .:.1 tein ele t ic; rc. a e d t e hi h, see ifie-s e mnetien r. sulting therefrom arise from the; d1 ;0P-.-ot,voltage, at the terminals oi -the furnace be lo ascertain critical value. .The same. degno ns r-a ion -indi;

a e t th ea s r r me ia s isult constituted by the, trough is, naturallypearthed,

whilst the other. electrode, ,|2 is connected by a. flexible conductor [6 and. an.ordinary.,.conductor I? to themovablebrush .I8 of; a. voltage regulator, comprising. a transformer; l9,.,th seeondary of which comprises.,a certain; numb e n,, .of voltage contactstuds 2 l co -operatingflyvith the brush, 18,, whilst the primary .22 of. the transiormeris conr. nected with themains in the ordinaryway The movable ,brush l 8 -is., .con t oiled ,by an elee ic mot-er uppl ed frqm;=. ..a xiliemlmeI 4. e T aQ I PP WE. e tween-25s.;m re: 19 12 2 hQJ QQiPQmZQ-Q s ai ceicaqr ey. id. lt elm e le e ithissfiz eynhere he... aerate; 38 o ;t e;eexiliern aat rmiaai sr he rm re 1a of the? re ay;..i .-ae ete nrai spring 3| in a position separated from-the CQlQ-f-j tact plug 21, whilst the-windingjzoi thisrelay tends, when it is excited, to place;the .armat u 1?e-, 28 in. contact with the plug 21,. Therelayin question is .of the voltmetric type that is :tosay the circuit or its winding 32 ;comprises.in le tion to a suitable source 33, the movablemember; 34 of an apparatus 35, such asasvoltmeter, in which the displacements ofthe movablemembep 34 are made dependent upon-the electricgpflten tial which is applied to the terminals of: this; apparatus.

The field circuit of the winding z32pfinallycom prises a plug 36 of adjustable position with respect to the movable member34 andcapable of; being contacted by this movable member whenj the voltage applied to the terminals .31 w0f ;.;the apparatus is below a limit value, determined; according to the position chosen for, theplug. 36. The terminals 31 areconnectedrespectively by conductors 38 and39 to, the trough-J0, and to the electrode I2 of the furnace The device also comprises a regulator ,of absorbed power of wattmetricrelay type, ThiS-;I'. lay comprises in the first .place. .atyp,e:0f; watt meter 4| inserted on the conductorJflandreceiv ing by an earthed conductor 42,;theinfluence of; the voltage applied to the electrodes. Its mov able member 43 co-operates withtwogadjustable contact plugs 44 and 44, as regardsposition and; which plugs are capable of being ,contacted,b the said movable member. Theseeontact plugs are inserted in two circuits c0mp1jsing -the ;-con ductor 24 of theauxiliary line, the windings gii and 45' of a relay, conductors 45, 45 andfi'l, the, conductor 29 of the auxiliaryline -and movable; member 43.

The windings of relays 4-5 and 45' in opposition control the same movable armature 48 connected, to the conductor 24 of the auxiliary line and reset by two springs 49 which, when any-of the windings 45 and 45' is not excited keep it in a medium position, between two fixed contact plugs; 5| and 5| connected by two conductors 52 and, 52 to the windings of an electric motor 53-wh-ich through the intermediationof speed-reducing gears 54 controls the rotation ot the winch 14s According to whether the.currentecirculatesin the conductor 52 or '52, the motor53 rotates int one direction or in the other. The circuit of-the. motor 53 is completed bye-conductor 55 which, terminates at a fixed contact plug-56 on which rests the movable armature 28 when it is freely; acted upon by its re-setting spring. 3!.

The operation of the device abovedescribedis; as follows:

The furnace bein supposed-as working the' movable member 34 of; the voltmetr-ic ap paratus5 35 is separated from the plug :36 sincethenormal; operating voltage is greater than the lower limit; voltage which has been fixed.

Furthermore, the movable member-43 of; the; wattmetric apparatus 4| is normally comprised between the plugs 44 and 44-, .so -that -none,oi the windings 45 and 45. of the relay-are excited and themotor '53 and the electrodefllz remain fixed.

However, if the voltage between the-electrodes I0 and I2 of the furnace drop below-the critical value, the movablemember 34,.contacts-the plug; 36, the winding 32 is excited lthe,armature-2& comes into contact with the plug zl; and the mp1; tor 23 which is thus supplied, ;;causes; ;the, d1s

.placement of the movable} brush latowards a 2' contact 2] of higher voltage, so that the movable member 34 of the voltmetric apparatus 35 leaves the plug 35 with the result that the motor 23 stops and this situation is maintained.

It is to be noted that during the operation of the'voltage regulator, the movable armature 28 leaves the plug 56' so that the supply circuit of the motor 53 is definitely open.

The regulation of the voltage being assured as has just been stated, the spring 3! brings the armature 28 into contact with the plug 56. If the 'wattmetric apparatus i! indicates that the power absorbed passes below or above the proper value, the movable member 43 contacts a plug M or i"which excites the Winding 45 or 45' corresponding theretoand closes the appropriate circuit of motor 53; the latter starts to rotate in the necessary direction and causes the raising or lowering of the electrode 92 until, the power absorbed having returned to its normal value, the movable member 43 cuts off the excitation of the winding 35 or 55' and causes the stoppage of the motor 53.

A modified form of device according to the present invention is shown in Figure 5. Most of the elements of this figure have already been illustrated in Figure 4 and like parts are indicated by like reference characters. In this form of construction, the conductors 33 and 35 extending respectively from the crucible lb and the electrode l2 are connected to terminals of voltmeter 68 or similar device sensitive to the potential and the movable element or pointer 61 of which travels over two sectors 32 and adjustable in position and disposed in the region of the scale of the voltmeter corresponding to the normal operating voltage of the furnace.

The pointer Ei is formed of conductive material and is connected by a conductor M to one of the poles of a source of current E5. The sector 62 is connected by a conductor 65 to a winding of a relay 6? connected On the other hand to the other pole of the source of current 65. Likewise, the sector s3 is connected by conductor 53 to a second winding of relay 69 similarly connected to the source 65.

The two windings of the relay 6'! and -69 are positioned in opposition and act in opposite directions on a movable armature iii which by means of two opposed springs 75 and i2 has a tendency to be maintained in a mean position equally distant from the two windings 61 and 69.

This movable armature which is connected permanently to a conductor 29 of the auxiliary line cooperates with three contact points l3, l4 and 75. Contact point 73 which is between the other two is that on which the armature 70 rests when neither of the windings 6i and 69 are excited. Contact point 73 is connected by the conductor 55 to the circuit of the motor 53 which moves the electrode l2.

Contact point id is engaged by the movable armature is when the winding 51 is excited. Similarly, when the winding 69 is excited, the armature it comes in contact with the contact point is. Contact points is and it are respectively connected by conductors 76 and ll to the inductor winding is and 79 of the motor 23 which actuates the movable arm is of the voltage regulator. Those windings are on the other hand connected by a common conductor 89 to the wire 24 of the auxiliary line. Depending upon whether the current circulates through the conductor 16 or 'I'Lthe motor 23 will turn in one direction or the other,

The relay which controls the motor 53 actuat ing the electrode i2 does not have'to'be a wattmetric relay, that is to say, sensitive to the power absorbed by the furnace. It is sufficient that this relay be sensitive to the intensity of the current absorbed by the furnace. In this respect, a simple ammeter Si is interposed in the conductor ll which feeds the furnace, but there is in this ammeter a movable element 33 which cooperates with the points to and 46 connected by conductors db and 55 to windings 15 and 45 already described in connection with Figure 4. Similarly, this movable element 43 is permanent- 1y connected by a conductor l? to the wire 29 of the auxiliary line.

The operation of this second form of apparatus is as follows: The furnace being in normal operation, the movable element 65 of the voltmetric apparatus $6 is positioned between the sectors 62 and 63 so that neither of the windings 61 or $9 is excited. The armature 18 is in contact with the point 73 and consequently the ammeter apparatus 8! can regulate the position of the electrode I2 according to the intensity of the current which is below or above the normal current.

If the potential between the electrodes l0 and I2 of the furnace falls below the critical value, the movable element ill will engage the contact 62, the winding iii will be excited and the armature I'll will come in contact with the point 74 which will excite the winding 19 of the motor 23 which turns and effects the displacement of the arm 58 toward a point 2i of high voltage. If the voltage increases and the movable element 6! of the voltmetric apparatus leaves the point 62, the motor 23 will be stopped and will maintain this position.

,Since during the operation the armature Til has left the point l3, the circuit of the motor 53 is broken and no displacement of the electrode I2 can take place.

If, by reason of irregularity in the electric supply network, the'voltage becomes too strong, the movable element til of the voltmetric apparatus'will engage the point 63 bringing the armature iii in engagement with contact 75. The motor 23 will then rotate in an opposite direction to reduce the voltage applied to the furnace to its normal value. Here, also during the operation, the circuit of the motor 53 is broken and the electrode i2 remains positively fixed.

I claim:

1. An apparatus for regulating the specific consumption of energy per ton of an electric arc furnace having spaced electrodes and conductors connecting the electrodes to a source of electrical energy comprising in combination means for varying the spacing between the electrodes, means sensitive to the power absorbed, for controlling the electrode spacing varying means, means for varying the potential of the. source of energy, and means responsive to the. potential existing between the electrodes for actuating the potential varying means.

2. An apparatus for regulating. the specific. consumption of energy per ton of an electric; arc furnace having spaced electrodes and con-- ductors connecting the electrodes to a source of electrical energy comprising in combination, means for varying the spacing between the electrodes, means responsive to the power absorbed for actuating said electrode spacing varying" means, a. regulator for the voltage of the source. of energy, and means sensitive to the potential";

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Reid Feb. 1, 1910 Number Number Number Name Date Creveling July 4, 1916 Montgomery et a1. Jan. 8, 1918 Lehr Oct. 28, 1919 Bulley July 27, 1920 Saklatawalla et a1. Jan. 8, 1924 Marshick May 27, 1924 Kenyon Feb. 2, 1946 Davis May 6, 1947 FOREIGN PATENTS Country Date Great Britain May 21, 1922 France Apr. 29, 1940 

